Conditions under which a constant velocity universal joint having a steel ball is used have become more diverse over recent years. The conditions are becoming more challenging in terms of, for example, increased temperature, increased load, and increased speed. Under such circumstances, temperature of the joint rises because of heat caused by sliding, temperature increase due to internal factors of bearings and joints, temperature increase due to external factors such as environment. In particular, in grease lubrication, the grease traps heat from the steel ball. Therefore, heat dissipation of the steel ball is lower compared to that of an inner joint component (inner ring) and an outer joint component (outer ring). As a result, the temperature of the steel ball increases significantly. The temperature increase in the steel ball becomes more significant particularly during high speed rotation. When the temperature increase in the steel ball is significant in this way, peeling of a surface layer of the steel ball occurs more easily compared to the inner and outer rings. Life of the constant velocity universal joint is shortened.
Therefore, conventionally, a joint is known in which surface roughness of the steel ball is made more similar to the surface roughness of a partner rolling surface (Patent Document 1). In other words, as a result of the surface roughness of the steel ball becoming more similar to the surface roughness of the partner rolling surface, lubrication between the steel ball and the inner and outer rings can be improved, and temperature increase of the steel ball can be suppressed. As a result, peeling of the surface layer can be prevented, and life of the steel ball can be extended.
Patent Document 1: Japanese Patent Laid-Open Publication No. Showa 61-167715